Not applicable.
Not Applicable.
The present invention relates generally to a method and apparatus for measuring the thickness of a layer of solids deposited on an inner pipeline surface. The thickness is determined from the Doppler frequency shift resulting from the local flow velocity at pre-determined distances away from the inner pipe wall. The present method and apparatus provide non-invasive, qualitative detection of solids buildup and quantitative measurement of thickness of solids on an inner pipe wall.
As the current trend in offshore oil and gas production advances into deeper waters, it is becoming increasingly necessary for the industry to develop cost-effective solutions for developing fields in deep and/or remote waters.
A typical solution for such cases is to keep the production facilities on a xe2x80x9chost platformxe2x80x9d and connect the deep-water well(s) to the platform with pipelines and risers. The supporting equipment for the subsea tree control, such as hydraulic and electric power units, chemical injection pumps and tanks, and a control console, are also housed on the host platform. The subsea tree control is accomplished via long umbilical(s) consisting of electric conductors, hydraulic lines and chemical injection lines laid alongside the pipeline. In addition, two parallel pipelines are necessary to accomplish the roundtrip pigging operations. The distance between the well and the host platform is known as the tieback distance. The cost and technical challenges of this type of conventional tieback system increase as the tieback distance increases, and to a lesser extent as the water depth increases. In most cases, 20 miles represents the practical limit for the maximum tieback distance with the conventional tieback system.
One limit on the length of subsea tiebacks conveying crude petroleum arises from flow assurance problems. Solids such as asphaltene and paraffin deposit on the inner walls of the tiebacks and partially, and in some cases completely, block the flow. The longer the tieback is, the greater the length of pipe that must be inspected and kept free of deposits.
At present, non-intrusive sensors that can adequately detect and characterize such deposits are not available. The present solutions require use of very expensive alternative methods for flow assurance, including twin flowlines (for round-trip pigging), heat traced or insulated tiebacks and pipelines. These alternative methods operate by attempting to prevent the deposition of solids on the flowline wall, and do not provide means for detecting the presence of solids in the event that deposits occur. The lack of continuous monitoring can result in undesirable shutdowns. For example, if a flowline has been kept clear by pigging at a certain frequency, e.g. once per month, and the composition of the fluid in the flowline changes so that deposits begin to form at a greater rate, the line will become clogged and possibly shut down because the previously established pigging frequency is now insufficient.
Some attempts have been made to provide systems for monitoring solids buildup. For example, monitoring devices such as that described in U.S. Pat. No. 4,490,679 identify paraffin by monitoring change in the resistance of an electromagnetic coil. The monitoring device requires access to the fluid and is housed in a recess in the pipe. It is desired to provide monitoring without disrupting the flow of fluid through the line and without requiring direct contact with the fluid.
Similarly, the optical asphaltene sensor described in U.S. Pat. No. 4,843,247 determines the content of asphaltene in heavy oils based on the absorption spectra of asphaltene. The invention uses visible light having wavelengths in the range of 500 nm to 1000 nm and thus requires at least optical transmission through the fluid, which is difficult to operate in a pipeline and prone to the coating of hydrocarbons on the optical windows that may distort the results.
There are other industrial applications in which it is desirable to measure thickness of a particular layer or object. For example, U.S. Pat. No. 5,929,349 discloses an ultrasonic inspecting tool that can be used to measure the thickness of a tank wall. U.S. Patent No. 5,092,176 provides a method for determining deposit buildup on the inner surface of a boiler tube by measuring sound attenuation in multiple echoes from the wall-deposit interface. To give an accurate measurement, the boiler tube must be empty of water so as to enhance the acoustic reflection by increasing the acoustic impedance mismatch between the deposit xe2x80x94air interface. If the tube is not emptied of fluid, the acoustic signals will be largely transmitted into the fluid in the tube and prevent accurate measurements. U.S. Pat. No. 5,734,098 provides a method for measuring mass deposition on the surface of a thickness-shear mode quartz resonator based on the resonance response of the piezoelectric crystal that is immersed in a fluid.
Conventional ultrasonic devices for measuring thickness are not always reliable for determining the thickness of a layer of deposited solids, however, because the acoustic impedance of the deposited material may be close to or even match the impedance of the adjacent liquid, with the result that little or no reflection is obtained from the surface of the deposited layer.
Hence, it is desired to provide a system that can operate over greater tieback distances without the cost and technical disadvantages that heretofore have prevented increasing the tieback distance. It is specifically desired to provide a method and apparatus for detecting deposits of asphaltene and paraffin on the inside wall of a pipeline without interrupting the flow of fluid through the pipeline. It is further desired to provide a robust system can measure the thickness of the deposited layer even if the acoustic impedance of the deposited layer and the Showing liquid match.
Another technique for measuring solids in a fluid passageway involves the use of Doppler backscattering. For example, Doppler backscattering techniques are used to monitoring tissue damage and to detect deposits in arteries. U.S. Pat. No. 5,657,760 to Ying et al. discloses a method of monitoring the extent and geometry of tissue that has undergone thermal treatment. The apparatus measures the phase change and amplitude of received echoes at multiple range depths to monitor the tissue thermal profile and geometry of tissue thermal damage. U.S. Pat. No. 5,327,893 to Savic discloses a method for detection of cholesterol deposits in arteries based on received acoustic signals with Doppler frequency shifts as result of artery heartbeat movement. The signals are analyzed by a pattern recognition technique that allows recognition of the artery with stenosis. U.S. Pat. No. 4,770,184 to Greene et al. provides a diagnostic system for monitoring human arteries based on comparison of the time-and frequency-domain Doppler signals of patients to those of normal people. These in vivo applications are different from the subsea invention in which it is currently desired to provide quantitative deposition measurement, based on measuring the Doppler frequency shifts resulting from the movement of the particulate in a fluid stream, and based on the known pipe wall thickness.
Hence, there is still a need for a reliable acoustic means for detecting and measuring wall deposition in subsea pipelines. In particular, the desired system should be robust enough to function in a subsea environment. The desired system should also be able to provide the desired measurements even when the acoustic impedance of the deposition layer and the flowing liquid match are the same or similar.
The present invention comprises a system including a method and apparatus for measuring the thickness of a layer of solids deposited on an inner pipeline surface. More specifically, the present system provides a method and apparatus for detecting deposits of asphaltene and paraffin on the inside wall of a pipeline. The present system can operate over extended tieback distances without incurring prohibitive costs and technical disadvantages. The present system is robust and can measure the thickness of the deposited layer even if the toustic impedance of the deposited layer and the flowing liquid match.
According to a preferred embodiment, the thickness of a deposited solid or semi-solid layer is determined from the Doppler frequency shift that results from the local flow velocity at a plurality of pre-determined distances from the inner pipe wall. The presence of deposited solids is detected by comparing the timing of the reflection of the inner surface of the pipe wall with the extra propagation time for the signal to reach flowing fluid. The thickness of the deposited layer is determined from the position of the innermost zero Doppler frequency shift, which may be extrapolated from the close-wall Doppler frequency shifts, and from the position of the inner pipe wall surface. The disclosed method and apparatus provide non-invasive, qualitative detection of deposition buildup and quantitative measurement of deposited solids on an inner pipe wall. Another embodiment of the invention comprises a method for monitoring and measuring the buildup of deposits on the inner surface of a pipeline containing flowing fluid, comprising: (a) transmitting a first acoustic signal into the pipeline through the pipeline wall, (b) receiving echoes of the transmitted signal, and (c) determining from the received echoes how far from the pipeline inner surface the interface between the deposits and the flowing fluid lies. Step (c) can include using the Doppler frequency shift of echoes from the flowing fluid to locate the deposit-fluid interface, and/or measuring the Doppler frequency shift of the echoes from at least two points in the flowing fluid and extrapolating the location of the zero velocity. The echo analysis can include time-gating the echoes. The transmitted signal need not be normal to the pipeline axis. In addition, more than one transmitter and/or more than one receiver can be used in the present method.
In still another embodiment, the present apparatus for monitoringand measuring the buildup of deposits on the inner surface of a pipeline containing flowing fluid, comprises: a first transmitter in acoustic communication with the pipeline wall and generating a transmitted signal, a first receiver in acoustic communication with the pipeline wall and generating received echo signal from a received signal comprising the reflection of the transmitted signal off the flowing fluid, and a microprocessor for determining from the received echo signal how far from the pipeline inner surface the interface between the deposits and the flowing fluid lies.